(1) Field of the Invention
The present invention relates to a solid-state imaging apparatus such as a CCD image sensor or the like and a method of driving the same, and in particular to a technique preventing white spots at the time of long-time accumulation.
(2) Description of the Related Art
Recently, the definition of a solid-state imaging apparatus has been increased up to 8 mega pixels or the like. This makes it possible to capture a still image as if it is taken by a silver halide camera, and take a video. A frame interline transfer solid-state imaging apparatus (hereinafter referred to as FITCCD) has been developed as a solid-state imaging apparatus in which so-called smear phenomenon rarely occurs.
A description is given below of the structure of a conventional FITCCD and a method of driving the FITCCD with reference to the drawings.
FIG. 1 is a diagram showing an enlarged view of an area of a FITCCD.
First, the structure of the FITCCD is described.
In FIG. 1, a solid-state imaging apparatus 100 includes: photodiodes 101 which are two-dimensionally arranged on a not-shown semiconductor substrate; a vertical CCD 102 for transferring signal electric charge accumulated in the photodiodes 101 in the vertical direction; an accumulation area (not shown) for accumulating the signal electric charge transferred by the vertical CCD 102; a horizontal CCD (not shown) for transferring the electric charge accumulated in the accumulation area in the horizontal direction; an output unit (not shown) for detecting the signal electric charge transferred by the horizontal CCD and outputting the signal electric charge; and a drain unit (not shown) for draining excess electric charge.
The vertical CCD 102 is composed of: a channel area; transfer electrodes (hereinafter referred to as also “read-out gates”) 105a, 105b, 105c and 105d each of which has a function as a read-out electrode for reading signal electric charge from the associated photodiode 101; and transfer electrodes (hereinafter referred to as also “non-read-out gates”) 104a, 104b, 104c and 104d each of which does not have a function as a read-out electrode for reading out signal electric charge from the associated photodiode 101.
Next, the conventional driving method is described with reference to the diagram of voltage waveforms shown as FIG. 2.
In FIG. 2, φ1 is a voltage pulse applied to each of the read-out gates 105a and 105c, φ2 is a voltage pulse applied to each of the non-read-out gates 104b and 104d, φ3 is a voltage pulse applied to each of the read-out gates 105b and 105d, and φ4 is a voltage pulse applied to each of the non-read-out gates 104a and 104c. 
In the conventional driving method, the voltage pulses φ1 and φ3 are at a Low (L) level and the voltage pulses φ2 and φ4 are at a High (H) level, for example, in the time period t4 during which electric charge is accumulated in the photodiodes 101. The time period t4 corresponds to a vertical time period and is the time during which electric charge is not read out. In other words, in the conventional driving method, the voltage pulses φ1 and φ3 are at the Low (L) level which is opposite in polarity to the High (H) level in the electric charge reading-out time period t2, for example, during the time period t4 in which electric charge is being accumulated in the photodiodes 101. The time period t4 corresponds to a vertical time period and is the time during which electric charge is not read out.
In this way, the potentials of the overlapped parts of the read-out gates 105a, 105b, 105c, and 105d come to undepleted states, and holes are accumulated immediately below the overlapped parts of the read-out gates 105a, 105b, 105c, and 105d. Accordingly, at the photodiode 101 and the overlapped parts of the read-out gates 105a, 105b, 105c, and 105d, dark currents are drastically reduced. Therefore, the quality of a reproduced image is improved.    Patent Reference Japanese Patent Publication No. 2851631
However, in the conventional driving method, when an increase in dark currents is prevented by accumulating holes immediately below read-out gates, over-accumulated holes unstabilize the potential of a semiconductor substrate, and excess electric charge is likely to inversely injected from the semiconductor substrate to photodiodes.
In other words, in a long-time accumulation mode (hereinafter referred to also as long-time accumulation mode) in which electric charge is accumulated in photodiodes during two or more vertical time periods, the potential of the semiconductor substrate is shifted due to the accumulated holes. Hence, white spots to be obstacles occur when reading out electric charge accumulated in the photodiodes. Excess electric charge to be obstacles when transferring the read-out electric charge is injected also to the channels of the vertical CCD.
The problem is noticeable particularly in the case of a refined CCD which is likely to induce an electric field concentration.